This invention relates to a metrological instrument, in particular a metrological instrument capable of measuring both surface form, that is the overall shape of a surface, and the texture or roughness of the surface.
The Form Talysurf Series series of metrological instruments produced by Taylor Hobson Limited of Leicester, England enable a three-dimensional surface area to be mapped and data to be displayed to the user showing the form and texture of that surface area.
In the Form Talysurf (Registered Trademark) Series of instruments this is achieved by causing a measurement probe to traverse the surface area to be measured along a measurement path in a first direction (generally the X direction), then moving a stage carrying the object whose surface is being mapped in a second orthogonal direction (generally the Y direction) and repeating these two steps until the entire area to be mapped has been covered and measurement data consisting of the data collected on a number of parallel measurement paths is obtained.
A graphical representation of the mapped surface area can then be displayed or printed out for a user. Generally, this display will include a perspective view of the three-dimensional surface. This graphical representation then enables the user to determine surface characteristics of the mapped area. For example, the graphical representation may enable a user to determine whether there are cracks or fissures in a sheet surface such as a sheet of steel.
Although such graphical representations enable a user easily to study and interpret surface features of nominally flat surfaces, interpretation of such graphical representations can prove more difficult where the surface being mapped has significant form, for example where the surface consists of or includes curved surfaces. To enable better visual interpretation of the graphical representation when the surface being measured has significant form, form fitting software has been developed and is supplied by Taylor Hobson Limited which uses a polynomial form fitting procedure to enable removal of the form of the surface from the measurement data so that the user can be provided with, for example, independent graphical representations of the overall form of the surface and of the surface texture of the surface.
Although this enables a user to be provided with a good visual indication of the overall surface form and/or surface texture of a desired surface, there is also a desire to be able to measure rather than visualise the three-dimensional form of the surface being measured.
It is an aim of the present invention to provide a metrological instrument that provides an improvement in the accuracy and repeatability of 3-D form measurements.
According to one aspect of the present invention, there is provided a metrological instrument having means for effecting relative movement between a measurement probe and a support surface in a first direction so as to cause the measurement probe to traverse and follow changes in a surface of an object mounted to the support surface, means for effecting relative movement between the support surface and the measurement probe in a second direction orthogonal to the first direction, control means for causing the measurement probe to be moved relative to the surface to be measured in the second direction after the measurement probe completes a measurement path in the first direction so that the measurement probe traverses a plurality of parallel measurement paths, and processing means operable to process measurement data received from the measurement probe as a result of the traversal of the plurality of parallel measurement paths, the processing means being operable to process measurement data received as a result of the measurement probe traversing a surface area of known three-dimensional form to determine the orientation of the second direction relative to the first direction and to provide data representing that relative orientation.
In another aspect, the present invention provides a metrology instrument having means for effecting relative movement between a measurement probe and a surface to be measured in two nominally orthogonal directions and means for calibrating one of the two nominally orthogonal directions relative to the other by using measurement data obtained by measurement of a reference surface of known three-dimensional form.